// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "util/hash.h"

#include "cstring"

#include "util/coding.h"

// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through
// between switch labels. The real definition should be provided externally.
// This one is a fallback version for unsupported compilers.
#ifndef FALLTHROUTH_INTENDED
#define FALLTHROUGH_INTENDED \
    do {                     \
    } while(0)
#endif

namespace leveldb {

uint32_t Hash(const char* data, size_t n, uint32_t seed) {
    // Similar to murmur hash
    const uint32_t m = 0xc6a4a793;
    const uint32_t r = 24;
    const char* limit = data + n;
    uint32_t h = seed ^ (n * m);

    // Pick up four bytes at a time
    while (data + 4 <= limit) {
        uint32_t w = DecodeFixed32(data);
        data += 4;
        h += w;
        h *= m;
        h ^= (h >> 16);
    }

    // Pick up remaining bytes
    switch (limit - data) {
        case 3:
            h += static_cast<uint8_t>(data[2]) << 16;
            FALLTHROUGH_INTENDED;
        case 2:
            h += static_cast<uint8_t>(data[1]) << 8;
            FALLTHROUGH_INTENDED;
        case 1:
            h += static_cast<uint8_t>(data[0]);
            h *= m;
            h ^= (h >> r);
            break;
    }
    return h;
}

}  // namespace leveldb
